I wonder what the best way to go about this. Almost all my planes want to climb up on applying throttle (worse upwind). So I give it a bit of down elevator to keep it level. In doing this I lose a bit of speed. I am thinking about mixing the elevator with the throttle but it is such a small correction I am not sure if my DX6i is capable of it.

Planes should climb when you apply power. A properly trimmed plane flies straight and level when at a given airspeed. Increasing the throttle (and thus the airspeed) climbs and decreasing the throttle descends.

Use your elevator trim to maintain altitude at various throttle settings. Any airplane will gain altitude as airspeed increases and lose it as airspeed decreases.Full scale pilots use elevator trim a lot.

Yes you will have to do this. This is the way all normal airplanes are flown. You trim for the airspeed you want (using a trim tab, changing the entire hstab, or the elevator), and then use throttle to make the plane sink, rise, or fly level.

Personally i'd fine tune the CG before adding downthrust or mixes. Climbing too strongly under power is a often symptom of being nose heavy (CG too far forward). only once you are 100% sure that the CG is in the optimal position should you start adjusting thrust angle.

High wing planes usually need a fair bit of downthrust even once the CG is nailed but on mid and low wing aerobatic types you should be able to get rid of excessive climbing purely by adjusting CG.

So far I have read standard model sport stuff ... but for racing you should be looking at having plane set-up as neutral as possible at WOT ... that means tailplane angle to main wing ... as little engine downthrust as possible ... all compensations are made in the design and incidence angles of the flying surfaces. We need to minimise induced drag and have power line as straight through model as possible.

A racer will descend when throttle is reduced needing elevator to maintain altitude ... should fly straight and level at WOT.

if it wants to pitch up that badly, it's likely to be all 3 of
a) severely overpowered compared to the design assumptions
b) excessively nose heavy cg
c) substantial longitudinal dihedral (sometimes incorrectly called decalage),where the wing is at a signficantly higher incidence angle than the stab.

If it was designed to be speed stable in a narrow range, like most trainers and low speed gliders you're going to have to fight it to go fast level.

Even fast acft need a lot of forward trim at high speed. A real-world example, P-51 based to boot: Reno racing Mustangs require full forward trim and then some stick force to maintain level at their top speed. When the elevator trim assembly failed on Galloping Ghost in 2011 the aircraft pitched up at 17.3g, blacking out the pilot and the acft crashed into the crowd. An earlier similar incident the pilot escaped - fortunately it pitched straight up at only 10g instead of rolling

Even fast acft need a lot of forward trim at high speed. A real-world example, P-51 based to boot: Reno racing Mustangs require full forward trim and then some stick force to maintain level at their top speed. When the elevator trim assembly failed on Galloping Ghost in 2011 the aircraft pitched up at 17.3g, blacking out the pilot and the acft crashed into the crowd. An earlier similar incident the pilot escaped - fortunately it pitched straight up at only 10g instead of rolling

Sorry (with deep respect for the Reno Pilots) ..... but a model racer can be set-up to be neutral at WOT - I know from my own model racing.

A model can be altered so easily by spacer under wing mount .... cut and reglue tail .... washers in motor mount ... with no requirement to pass an Aeronautical Official Inspection ....

My RM Racer was reasonably successful and was zero trim at WOT ... I'd LOVE to have that bird again ... but passed it to another guy when I left UK.